In recent years, field-effect transistors using a nitride semiconductor, particularly, GaN-based transistors, are expected to be applied to devices with a higher withstanding voltage and a higher output, also known as power devices, employed in server systems, for example, in view of their physical characteristics. They are also expected to be applied to high-output amplifiers employed in base stations, such as wireless base stations, for their lower power consumptions.
In addition, efforts have been made for research and development of GaN-based transistors having a metal insulator semiconductor (MIS) structure. An MIS structure includes a gate insulation film between agate electrode and a semiconductor layer, for the purpose of suppressing leak currents, from the gate electrode, which adversely affect the characteristics of the GaN-based transistor.
In order to further improve the characteristics of GaN-based transistors, GaN-based transistors capable of operating under higher currents or in a high-temperature environment are needed to be developed. Currently, a GaN-based transistor includes electrodes including an Al layer, as ohmic electrodes, and further includes interconnections made of gold (Au) which is a low-resistance interconnection material, as interconnections. When a GaN-based transistor in such a structure is operated with a direct contact of the Au interconnection and the Al layer, an Au—Al compound is readily formed, which causes an increase in the resistance. For preventing this phenomenon, provision of a single Pt layer, Ta layer, TaN layer, TiWN layer and so forth, as a barrier metal layer, has been proposed, between the Au interconnection and the Al layer.